1. Field of the Invention
The present invention relates to hearing aids as well as methods for the operation of hearing aids of the type having a microphone system for picking up an acoustic input signal and for emission of a microphone output signal, a signal processing unit, and an output transducer for emission of an output signal.
2. Description of the Prior Art
Modern hearing aids use devices for classification of hearing situations. The transmission parameters of the hearing aid are automatically varied depending on the hearing situation. In this case, the classification may, inter alia, influence the method of operation of the interference noise suppression algorithms, and of the microphone system. Thus, for example, depending on the identified hearing situation, a choice is made (by discrete switching or by continuous overlaying) between an omnidirectional characteristic (zero directional characteristic) and significant directionality of the microphone system (first or higher order directional characteristic). The directional characteristic is produced by using gradient microphones or by electrically connecting a number of omnidirectional microphones to one another. Microphone systems such as these have a frequency-dependent transmission response, which is characterized by a considerable fall at low frequencies. The noise behavior of the microphones, on the other hand, is not dependent on the frequency, and is slightly amplified in comparison to an omnidirectional microphone. In order to achieve a natural tonal impression, the high-pass frequency response of the microphone system must be compensated for by amplification of the low frequencies. In the process, the noise that is present in the low frequency range is likewise amplified and, in some circumstances, is significantly audible in a disturbing manner, while quiet sounds are masked by the noise.
German OS 101 14 101 discloses a method for processing an input signal in a signal-processing unit in a hearing aid. One embodiment of the known hearing aid has two microphones, with a delay element being connected to one microphone, the delay of which is set as a function of the result of a modulation analysis, in order to improve the signal processing and to reduce the interference noise.
PCT Application 00/76268 discloses a hearing aid having a signal processing unit and at least two microphones, which can be connected to one another in order to form directional microphone systems of different order, in which case the directional microphone systems may themselves be connected to one another with a weighting which is dependent on the frequency of the microphone signals emitted from the microphones. The cut-off frequency between adjacent frequency bands in which a different weighting is provided for the microphone signals can be set as a function of the result of a signal analysis.
European Application 0 942 627 discloses a hearing aid having a directional microphone system with a signal processing device, an earpiece and a number of microphones, whose output signals can be connected to one another via delay devices and the signal processing device, with different weightings, in order to produce an individual directional microphone characteristic. The preferred reception direction (main direction) for the directional microphone system can be set individually to match an existing hearing situation.
U.S. Pat. No. 5,524,056 discloses a hearing aid having an omnidirectional microphone and having a first or higher order directional microphone. The low signal frequency range in the microphone signal from the directional microphone is amplified, and is matched to the microphone signal from the omnidirectional microphone. Both the microphone signal from the omnidirectional microphone and the microphone signal from the directional microphone are supplied to a switching unit. When the switching unit is in a first switch position, the omnidirectional microphone is connected to a hearing aid amplifier, and when the switching unit is in a second switch position, the directional microphone is connected to a hearing aid amplifier. The switching unit can switch automatically as a function of the signal level of a microphone signal.
The known hearing aids with a directional microphone system have the disadvantage that, in certain hearing situations, either the directionality of the microphone system is not optimally used, or a high degree of directionality leads to a clearly audible degradation in the tonal quality. In particular, when the level of the acoustic input signal is low, the signal-to-noise ratio becomes worse, and a hearing aid wearer perceives this in the form of disturbing microphone noise in a quiet environment.